Nutrient concentration-Flow relationships and loads in the South Pine River, South-eastern Queensland. I. Phosphorus loads

Abstract

Total P (TP), particulate P (PP), and total dissolved P (TDP) were monitored over 24 months in the South Pine River, south-eastern Queensland. The sampling interval ranged from 13-16 days during baseflow to less than 30 min duling stormflow. Baseflow TP concentrations were relatively constant at 0.03 mg L^-1. During stormflow, concentrations of both TDP and PP increased significantly and were positively correlated with flow. TP loads were 8950 kg (56.7 kg km^-2 year^-1) and 3980 kg (25.2 kg^-1 year^-1) in 1984 and 1985 respectively. Annual load was influenced by both the absolute discharge volume and the relative proportion of stormflow. PP was the dominant P fraction, comprising 77% of the TP load. Approximately 86, 89 and 75% of the TP, PP and TDP loads, respectively, were carried in only 2.8% of the time (20 days) during three major storms (50.8% of total flow). Concentrations and exports were high relative to those reported in other Australian studies, suggesting that climatic factors, particularly rainfall intensity, are important in determining export.

As a result of the variability in unit area export attributable to variations in runoff, unit area export coefficients are of negligible use for predictive purposes. However, by expressing load in terms of mass per unit runoff per unit area (kg mm^-1 km^-2), variance attributable to discharge is removed. When expressed in this form, stormflow export was relatively constant (0.46-0.54 kg mm^-1 km^-2). A method of predicting load from the flow record using this coefficient is proposed.